Hi Rowan. I can confirm that a pass limit set to 30 gives an end result with approximately same time and end noise level. BUT the noise calculations up till this is highly weird when the HQ denoiser is on :(
Test 1 - Normal filtering, no denoiser, 30 pass limit
reported noise level after 5 passes: 7,17%
reported noise level after 10 passes: 3,58%
reported noise level after 15 passes: 2,72%
reported noise level after 20 passes: 2,29%
reported noise level after 25 passes: 2,02%
reported noise level after 30 passes: 1,84%
Test 2 - Normal filtering, HQ denoiser, 30 pass limit
reported noise level after 5 passes: 21,06%
reported noise level after 10 passes: 10,11%
reported noise level after 15 passes: 7,45%
reported noise level after 20 passes: 6,21%
reported noise level after 25 passes: 5,44%
reported noise level after 30 passes: 1,82%
It seems to me, that there's something wrong with the noise calculation during rendering when the HQ denoiser is on, no matter what limit type (noise or passes) you use. The jump between the noise levels at 25 passes and 30 passes is simply unrealistic.
Hope this helps
Edit: Just made a quick test showing, that when using Intell AI denoiser, the reported noise levels during rendering matches the reported levels with no denoiser. So ATM Intell AI is the Denoiser to use when rendering with noise level limit.